Our overall goal is to define some of the parameters which determine the capacity of lymphoid cells and tumor cells to serve both as stimulators for in vitro mixed lymphocyte reactions and as stimulators for the in vitro generation of cytotoxic T cells; then to analyse the interactions among subpopulations of responder cells as they react to various stimulator products--either membrane antigens or other "factors" produced by the stimulator cells. We are determining the rate of Ia antigen turnover on various B cell and tumor cell populations, separated by size or source, and comparing these turnover rates with those of other membrane proteins including H-2D and H-2K. In addition, we are determining what influence various agents have on membrane turnover and the subsequent stimulatory capacity these treated cells exhibit in mixed leukocyte cultures. Agents to be used include inhibitors of proteolytic enzymes, membrane cross-linking agents, and inhibitors of protein synthesis.